The present invention relates to a dust collector and method for collecting dust, which is used to remove dust, mist, and the like contained in a gas.
In order to efficiently collect fine dust (submicron particles), mist, and the like, the applicant has before proposed a dust collector in Japanese Patent Provisional Publication No. 10-174899 (No. 174899/1998).
This dust collector includes charging means for charging a substance to be collected such as dust and mist contained in a gas, spray means for spraying a dielectric on the substance to be collected charged by the charging means, electric field forming means for forming an electric field for dielectrically polarizing the dielectric sprayed from the spray means, and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected.
The above-described dust collector has a high voltage applied electrode 100 and a ground electrode 200, shown in FIG. 24, as the electric field forming means, and allows an exhaust gas containing the substance to be collected such as dust and mist (in this example, SO3 mist indicated by the black dots in the figure) 300 and a dielectric (in this example, water mist) 400 sprayed from the spray means to flow between the electrodes 100 and 200.
The substance to be collected 300 has been charged, for example, negatively in advance by the charging means. On the other hand, the dielectric 400 is dielectrically polarized by a direct current electric field formed between the electrodes 100 and 200. Therefore, the substance to be collected 300 is collected by the dielectric 400 by means of the Coulomb""s force acting between the particles of dielectric 400.
When an alternating voltage is applied between the electrodes 100 and 200 as shown in FIG. 25, the polarization polarity of the dielectric 400 changes with time, and the charged substance to be collected moves in a zigzag form. Thus, the substance to be collected 300 is collected by the dielectric 400 by means of the Coulomb""s force acting between the particles of dielectric 400.
According to this dust collector of the earlier application, submicron particles can be collected efficiently despite the compact configuration.
In order to further increase the efficiency in collecting the substance to be collected 300, it is necessary for the dielectric 400 to exist enough up to the upper part (rear part) of the electrodes 100 and 200. In the conventional collector, however, the dielectric shows a tendency to rarefy at the upper part (rear part) of the electrodes 100 and 200.
The inventors found that the aforementioned tendency is ascribed to the charging of the dielectric sprayed from the spray means.
Specifically, the particles of dielectric sprayed from the spray means are charged positively or negatively because the particles of dielectric exchange charges at the boundary of a pipe through which the dielectric it self flows. Therefore, the dielectric 400 having been charged positively or negatively is sprayed from the spray means, which is a cause of bringing about the aforementioned tendency as described below.
In FIG. 26 corresponding to FIG. 24, the circle mark applied to the side of the particle of dielectric 400 indicates the charging state of the particle of dielectric 400. If the charged dielectric 400 is supplied between the electrodes 100 and 200, the positively charged dielectric 400 is attracted to the electrode 100, and the negatively charged dielectric 400 is attracted to the electrode 200 by means of the Coulomb""s force. Therefore, most of the dielectric 400 is collected by the electrodes 100 and 200 before it arrives at the upper part (rear part) of the electrodes 100 and 200.
FIG. 27 shows a case where an alternating electric field is applied to between the electrodes 100 and 200. In this case, the charged dielectric 400 goes while being swayed to right and left with the change cycles of alternating electric field. At this time, the particles of dielectric 400 having a positive and negative charge are attracted to one another and aggregate, so that the distribution concentration of the dielectric 400 decreases toward the upper part of the electrodes 100 and 200. That is, even if an alternating electric field is applied to between the electrodes 100 and 200, the dielectric 400 rarefies at the upper part of the electrodes 100 and 200.
The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a dust collector and method for collecting dust in which the rarefaction of dielectric at the rear part of electric field forming means is prevented, whereby the collecting efficiency can be increased.
To achieve the above object, the present invention provides a dust collector, comprising charging means for charging a substance to be collected, such as dust and mist, contained in a gas; spray means for spraying a dielectric on the substance to be collected charged by the charging means; electric field forming means, having first and second electrodes for forming a direct current electric field, for dielectrically polarizing the dielectric sprayed by the spray means by means of the direct current electric field; dielectric collecting means for collecting the dielectric which has arrested the substance to be collected; and grounding means, provided in the spray means, for electrically grounding the dielectric before being sprayed, wherein a charge of the dielectric is caused to escape by the grounding means so that the dielectric is made electrically neutral.
According to the present invention, since the electrically neutral dielectric is sprayed from the spray means, the arrest of the sprayed dielectric by the electrode of the electric field forming means is restrained. Therefore, a shortage of dielectric in the rear zone of an electric field forming section is prevented, so that the efficiency in collecting the substance to be collected is increased.
A metallic net is used as the grounding means, and the net can be disposed in a flow path of the dielectric in the spray means so as to traverse the flow path. With the use of the metallic net as de-electrifying means, a satisfactory de-electrifying effect can be achieved without obstructing the flow of the dielectric.
Also, the present invention provides a dust collector, comprising charging means for charging a substance to be collected, such as dust and mist, contained in a gas; spray means for spraying a dielectric on the substance to be collected charged by the charging means; electric field forming means, having first and second electrodes for forming a direct current electric field, for dielectrically polarizing the dielectric sprayed by the spray means by means of the direct current electric field; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein a plurality of corona discharge sections arranged in the flow direction of the gas at given intervals are formed on the opposed surfaces of the first and second electrodes to generate band-shaped uniform corona discharge perpendicular to the gas flow, and the dielectric is provided with a charge of reverse polarity alternately by the corona discharge.
According to the present invention, the dielectric goes in a zigzag form to the rear zone of the electric field forming means under the action of the charge developed by discharge of the corona discharge section, so that the substance to be collected can be collected very efficiently.
The arrangement interval between the corona discharge sections on the first electrode and the arrangement interval between the corona discharge sections on the second electrode are preferably set so as to be equal to each other. Also, both of the corona discharge sections are preferably provided so as to have an arrangement phase difference of xc2xd of the arrangement interval in the flow direction of the gas. According to this configuration, corona discharge on the electrodes of the electric field forming section does not oppose, so that the occurrence of spark discharge can be restrained.
The rear parts of the first and second electrodes can be extended, and a plurality of the corona discharge sections can be formed in the flow direction of the gas on one of these extensions only. According to this configuration, the dielectric can be collected at the extension of the electrode of the electric field forming section, so that a demister can be omitted.
Further, the present invention provides a dust collector, comprising charging means for charging a substance to be collected, such as dust and mist, contained in a gas; spray means for spraying a dielectric on the substance to be collected charged by the charging means; electric field forming means, having first and second electrodes for forming a direct current electric field, for dielectrically polarizing the dielectric sprayed by the spray means by means of the direct current electric field; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein the distribution of the dielectric sprayed by the spray means is set so that the distribution of the dielectric at the rear part of the first and second electrodes is uniformed.
According to the present invention, the dielectric can be caused to exist uniformly in the rear zone of the electric field forming section, so that the collecting efficiency is increased.
Sill further, the present invention provides a dust collector, comprising charging means for charging a substance to be collected, such as dust and mist, contained in a gas; spray means for spraying a dielectric on the substance to be collected charged by the charging means; electric field forming means, having first and second electrodes for forming a direct current electric field, for dielectrically polarizing the dielectric sprayed by the spray means by means of the direct current electric field; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein the spray means is provided with charge providing means for providing the dielectric before being sprayed with a charge having a reverse polarity of the charging polarity of the substance to be collected.
According to the present invention, a repelling force acts between the particles of sprayed dielectric, so that the aggregation of the particles of dielectric in the electric field forming section is prevented, thereby increasing the collecting efficiency.
The charge providing means can be configured so as to supply ionized air to the dielectric before being sprayed. According to this configuration, the dielectric is charged via the ionized air.
Also, the charge providing means can be configured so that magnetism in the direction perpendicular to the flow direction of the dielectric is applied to the dielectric before being sprayed. According to this configuration, the dielectric is charged by the action of the magnetism.
In the dust collectors described above, a plurality of stages of the pair of the spray means and the electric field forming means can be disposed. According to this configuration, the substance to be collected is collected in a dust collecting section of each stage, so that a very high dust collecting efficiency can be obtained.
In this configuration, fresh water is sprayed from spray means of at least the most downstream stage of the plurality of spray means, and circulating water is sprayed from spray means excluding the spray means which sprays fresh water. According to this configuration, since fresh water is sprayed from spray means of at least the most downstream stage, the collecting efficiency is further increased. Therefore, this configuration is especially advantageous in preventing the outflow of harmful substances.
The spray means of the most downstream stage can be provided with a nozzle for atomizing the fresh water to an average diameter not larger than 50 xcexcm. If such a nozzle is provided, the nozzle is not clogged, thereby maintaining a high dust collecting efficiency, and the quantity of fresh water used can be decreased.
The dust collectors described above can be configured so as to further comprise a dielectric circulating system for supplying the dielectric from a dielectric storage tank to the spray means and for returning the sprayed dielectric from the spray means to the storage tank; dielectric supply means for supplying a fresh dielectric to the dielectric storage tank; dielectric discharge means for discharging the dielectric in the dielectric storage tank; absorbent charging means for charging an absorbent in the dielectric storage tank, the absorbent being used to absorb a reaction product produced by a substance in the gas; and control means for controlling the quantity of dielectric supplied by the dielectric supply means and the quantity of dielectric discharged by the dielectric discharge means so that the concentration of the reaction product exhibits a value within a given range and for controlling the quantity of absorbent charged by the absorbent charging means so that the pH value of the dielectric exhibits a value within a given range.
According to this configuration, the deterioration in dielectric can be prevented, and also harmful gas can be absorbed and removed positively.
A method for collecting dust in accordance with the present invention comprises a first step of charging a substance to be collected, such as dust and mist, contained in a gas; a second step of causing the gas having undergone the first step to flow from the downside to the upside; a third step of spraying a dielectric on the substance to be collected contained in the gas flowing from the downside to the upside; a fourth step of dielectrically polarizing the sprayed dielectric and of causing the dielectric to arrest the substance to be collected by means of the Coulomb""s force created by the polarization; and a fifth step of collecting the dielectric which has arrested the substance to be arrested.
According to the present invention, the gas in which the substance to be collected has been charged is moved from the downside to the upside, so that a nonuniform distribution of the substance to be collected caused by the action of the gravity is not formed. Therefore, the substance to be collected is distributed uniformly, and is collected efficiently.